The present invention is directed towards a vehicle position determination system and method for determining the position of a moving vehicle having a transponder.
In electronic toll systems which monitor adjacent lanes of traffic flow, Automatic Vehicle Identification ("AVI") Radio Frequency ("RF") coverage is often used to provide coverage zones that extend from side-to-side of each lane. RF transponders mounted on the vehicles using the toll system are interrogated by AVI RF antennas that are connected to a roadside AVI reader. In toll systems, the roadside AVI reader is typically connected to a lane controller which is also connected to a vehicle detector and an imaging system which works in association with the AVI RF system to permit the vehicles to be detected, classified, and photographed, and the license plate numbers analyzed in order to permit the operator of the toll system to apply appropriate charges to the owner of the vehicle. In electronic toll systems, it is often necessary to determine in which lateral position a vehicle is traveling. For example, it is often necessary to separate vehicles equipped with transponders from vehicles without transponders and associate video images with vehicles that are not equipped. In order to do so, the electronic toll system must clearly identify where the subject vehicle is located within the multiple zones of coverage.
In previous systems, the location of the transponder and vehicle is subject to uncertainty because the RF capture zones may overlap. In open road applications, the overlap provided is usually extensive by design in order to provide shoulder-to-shoulder coverage of the entire roadway. In such systems, the roadside AVI reader establishes communications with the transponder in any one of several overlapping zones, and accordingly the system cannot ascertain in which lane the subject vehicle is located by using communications as the only location method. One attempt to solve this problem uses separate coverage zones to cover each of the adjacent lanes, with a further trailing coverage zone to cover vehicles that might drive along the center of the road. In such previous systems, the trailing center capture zone could not be placed co-linear with the capture zones for the adjacent lanes as it was necessary to differentiate the transponder data available from the center zone from the other capture zones in order to allow the roadside equipment to differentiate between vehicles correctly positioned within their respective lanes, and vehicles straddling the center line between two lanes. Such a configuration can result in time ambiguity as the data from the trailing coverage zone is received later than the data from the other coverage zones, during which time the co-ordination with the vehicle detector and lane controller is made ambiguous and the vehicle may have changed positions.
Overlapping coverage zones can also be problematic in conventional toll plaza or toll gate systems. In such systems, the lanes are typically separated by physical barriers, and accordingly the overlap between adjacent coverage zones is generally small. Nonetheless, the small amount of overlap that does exist can result in a transponder equipped vehicle being processed in the wrong lane. When a transponder is processed in the wrong lane, the normal high speed processing of AVI transactions is severely disrupted because the patron who is charged incorrectly will not receive an indication to proceed through the lane.
It is therefore desirable to provide a vehicle position determination system and method having improved accuracy for determining the position of a moving vehicle having a transponder in an electronic toll system. It is also desirable to provide such a system in which all the RF coverage zones can be aligned co-linearly across a roadway in order to minimize time ambiguity between the zones.